Muscular contraction reflexly relaxes tracheal smooth muscle in dogs

Although contraction of hindlimb skeletal muscle is well known to reflexly increase ventilation, heart rate and arterial pressure, little is known about the reflex effect of this maneuver on airway smooth muscle tone. Therefore, in chloralose-anesthetized dogs, we recorded transverse tension from the trachealis muscle while we constracted both gracilis muscles by electrically stimulating the gracilis nerves at 5 and 40 Hz. In 11 of the 13 dogs studied, static (40 Hz) contraction decreased tracheal tension, whereas in the remaining 2 dogs, static contraction increased tension. In 9 of 11 dogs, rhythmic (5 Hz) contraction decreased tracheal tension, whereas in the remaining 2, this maneuver increased tension. The changes in tracheal tension induced by stimulating the gracilis nerves were abolished by paralyzing the dogs and were restored, for the most part, after paralysis had dissipated. In addition, the contraction-induced changes in tension were not present when the gracilis muscles were contracted by stimulating the cut peripheral ends of the gracilis nerves. We conclude that muscular contraction reflexly relaxes tracheal smooth muscle in most dogs.